1. Field of the Invention
This invention relates to an image scanner designed for high-speed image scanning.
2. Description of the Related Art
In most of the present electronic-parts mounting systems, before the electronic components picked up from the parts feeder are mounted on an object such as a board or leadframe, they are observed by a scanner, and their positional deviations from ideal positions are detected by an image processor and then corrected. As this image scanner, there are two types, one of which reads a whole image in a two-dimensional way, and the other one of which reads the whole image in a one-dimensional way in a primary scanning direction perpendicular to the movement of an electronic component and accumulates the read one-dimensional images in a secondary scanning direction to form a two-dimensional image. This invention employs the latter type.
The principle on which a one-dimensional CCD camera operates will be described before we mention the drawbacks of the prior art. FIGS. 12 and 13 are diagrams for use in explaining the operation of the one-dimensional CCD camera. In FIG. 12, reference numeral 1 represents a one-dimensional CCD camera. The one-dimensional CCD camera 1 has an photosensitive element array 2 which is formed of photosensitive elements for storing charges according to the intensity of received light. These photosensitive elements are arranged in a line from one end to the other end of the photosensitive element array 2. The camera 1 also has a shift register 3 formed of elements facing the photosensitive elements one to one and receiving the charges transferred at a time from the photosensitive element array 2, and a shift gate 4 which permits or inhibits the transfer of the charges from the photosensitive element array 2 to the shift register 3. The shift gate 4 normally inhibits the transfer, and during this time the charges are stored in the respective photosensitive elements. When a transfer command signal is supplied to the shift gate 4, the charges of the photosensitive elements are simultaneously transferred to the corresponding elements of the shift register 3. The charges transferred to the shift register 3 are sequentially taken out, starting from the charge of the photosensitive element of the one end, as an image signal through an output portion 5 in synchronism with a clock signal.
Here, it is assumed that when the charges in the photosensitive elements of the array 2 are represented by A1, A2, . . . , Anxe2x88x921, An, the transfer command signal is supplied to the shift gate 4. At this time, the charges A1, A2, . . . , Anxe2x88x921, An are transferred to the shift register 3 at a time, and then the image signal is sequentially output in synchronism with the clock signal as indicated by an arrow N1 in FIG. 13. If the transfer command signal is supplied to the shift gate 4 before the image signal is completely produced as shown in FIG. 13, the charges remaining in the elements of the shift register 3 on the output portion 5 side are mixed with those transferred from the photosensitive element array 2, and as a result an error signal is produced. Specifically, as shown in FIG. 13, new charges B1, B2, . . . , Bnxe2x88x921, Bn are already accumulated in the photosensitive elements of the array 2, and if the shift gate 4 permits the transfer, the charges An, . . . , Anxe2x88x924 will be mixed with charges B5, . . . , B1, respectively. Therefore, the transfer command signal cannot be supplied to the shift gate 4 until all the charges of the shift register 3 are completely sent to the outside. This means that the read time depends on the number of elements of the shift register 3, that is, the number of photosensitive elements of the array 2.
FIG. 14 is a diagram for use in explaining the relation between a visual field to be fixed and electronic components in a conventional image scanner. In FIG. 14, a symbol S1 represents an image of a large-size component, for example, a large QFP (quadrilateral flat package), and S2 an image of a small-size component, for example, a rectangular chip. As illustrated in FIG. 14, the length WO of the photosensitive element array 2 is selected to be very large so that even if the large-size component is somewhat deviated in position, the edges of the component never stick out from the visual field V1. For the above reason, it takes a long time to read the line in the primary scanning direction. Therefore, the conventional image scanner has the drawback that it cannot operate at high speed since it takes a long time to read the image signal.
It is an object of the present invention to provide an image scanner and method of image scanning capable of high-speed scanning by reducing the image reading time when the image of a small-size electronic component is scanned.
According to the present invention, there is provided an image scanner including a photosensitive array having a plurality of photosensitive elements arranged in a row from one end to the other end which are exposed to light from an object to accumulate a plurality of charges therein, a shift gate for receiving the plurality of charges stored in the plurality of photosensitive elements and simultaneously transferring the plurality of charges in accordance with a transfer command signal, a shift register for receiving the plurality of charges simultaneously transferred from the shift gate, and sequentially supplying the plurality of charges one by one, starting from the charge stored in the photosensitive element of the one end, and transfer command signal output means for supplying the transfer command signal to the shift gate, the output means being constructed to be able to change the intervals of time at which the transfer command signal is supplied.
Moreover, according to the present invention, there is provided an image scanning method for reading an image by a photosensitive element array which has a plurality of photosensitive elements arranged from one end to the other end, including the steps of blocking a certain number of photosensitive elements of the plurality of photosensitive elements sequentially arranged starting from the other end of the photosensitive element array so that charges cannot be accumulated in the certain number of photosensitive elements, exposing the other ones of the plurality of photosensitive elements to light so that charges can be accumulated in the other photosensitive elements, simultaneously transferring a plurality of charges stored in the other photosensitive elements to a shift register, sequentially supplying the plurality of transferred charges from the shift register one by one starting from the charge stored in the photosensitive element of the one end, and simultaneously transferring new charges accumulated in the other photosensitive elements to the shift register after all the plurality of charges are completely fed from the shift register.
According to the present invention, an interval of time, at which the transfer command output means outputs the transfer command signal, can be changed in accordance with the size of an object to be imaged, such as an electronic component, that is, the size of an image to be obtained. Therefore, when the image of a small-size object to be imaged is scanned or read out, the output interval of the transfer command signal can be reduced and the image can be fast read out. When the image of a large-size object is read out, the output interval of the transfer command signal can be extended and the visual field can be widened.